The major histocompatibility complex (MHC) Class I antigens are expressed on virtually all types of vertebrate cells examined. These highly polymorphic transmembrane glycoproteins have a 45 kD heavy chain consisting of a short cytoplasmic C-terminal tail, a transmembrane region, and an extracellular N-terminal sequence which encompasses three domains (.alpha..sub.1, .alpha..sub.2, and .alpha..sub.3). .alpha..sub.1 and .alpha..sub.2 -domains carry all the immunological polymorphism, while the membrane-proximal .alpha..sub.3 domain is non-covalently associated with the 12 kD .beta..sub.2 microglobulin.
The MHC Class I antigen plays an essential role in restriction of the target cell repertoire of cytotoxic T-lymphocytes (CTL). Restriction involves preferential utilization of the different polymorphic MHC Class I antigens, H-2K, -D or -L (for mouse) or HLA-A, -B, or -C (for human), e.g. in recognition of vitally infected cells. For the most part, attention has been directed to the role of the MHC Class I antigens in restricting T-cell activity. However, in addition to restriction of the immune response, Class I MHC antigens may also interact with receptors expressed on the cell surface.